An island or isle is a piece of subcontinental land completely surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. Sedimentary islands in the Ganges Delta are called chars. A grouping of geographically or geologically related islands, such as the Philippines, is referred to as an archipelago.

There are two main types of islands in the sea: continental islands and oceanic islands. There are also artificial islands (man-made islands).

There are about 900,000 official islands in the world. This number consists of all the officially-reported islands of each country. The total number of islands in the world is unknown. There may be hundreds of thousands of tiny islands that are unknown and uncounted. The number of sea islands in the world is estimated to be more than 200,000. The total area of the world's sea islands is approx. 9,963,000 sq km, which is similar to the area of Canada and accounts for roughly 1/15 (or 6.7%) of the total land area of Earth.